1. Field of the Invention
The present invention relates to a method of forming an antifouling or antibacterial layer containing metal-modified apatite. The present invention also relates to an electronic device partially or entirely coated with such a layer.
2. Description of the Related Art
Portable electronic devices, in particular cellular phones, may always be carried by the users and frequently touched for operation, in comparison with much heavier household appliances such as washing machines or refrigerators. Eventually, particular parts of the portable devices will become dirty by the sebum of the user's hands and further by the dust accumulated on the sebum. If the user is a smoker, the device may also be contaminated by the tar of cigarettes. Unfavorably, these accumulated dust or cigarette tar often gives rise to the proliferation of bacteria at the contaminated portion of the device.
In light of the above, recent portable electronic devices are often subjected to antibacterial treatment before they are put on the market. The desired antibacterial function can be provided by using a photocatalytic semiconductor in producing an outward coating of the device.
In a photocatalytic semiconductor, an electron can make a transition from the valence band to the conduction band when the semiconductor absorbs radiation having an energy corresponding to the band gap between the valence band and the conductor band. Due to the transition, a positively charged hole occurs in the valence band. When a pollutant comes into contact with the semiconductor, the electron present in the conduction band will move onto the pollutant, thereby reducing the pollutant. Then, the hole in the valence band strips the electron from the pollutant, to oxidize the pollutant.
When TiO2 is used as a photocatalytic substance, the transit electron in the conduction band reduces the oxygen in the air, thereby producing a super-oxydo-anion (O2−). On the other hand, the hole occurring in the valence band oxidizes water present on the surface of the TiO2, thereby producing a hydroxy radical (OH), which is a very strong oxidative molecule. Thus, while an organic compound is held in contact with the TiO2, it will be decomposed to water and carbon dioxide. Such an organics-degrading material as TiO2 is widely used for antibacterial or antifouling purposes.
Japanese patent application laid-open No. 11(1999)-195345, for example, discloses an antibacterial technique in which photocatalytic TiO2 is applied to the push buttons of an electronic device. Unfavorably, TiO2 has a weak adsorbing power with the contaminants. Thus, to put the antibacterial or antifouling function of the TiO2 to effective use, the contaminants need to be held in proper contact with the TiO2.
Japanese patent application laid-open No. 2000-327315 discloses a contact-improving technique between contaminants and TiO2. Specifically, the JP document teaches that TiO2 is combined, on the scale of atoms, with highly adsorptive calcium hydroxy apatite (CaHAP, or Ca10(PO4)6(OH)2), to provide metal-modified apatite. The metal-modified apatite has such a structure as obtained by partially replacing Ca contained in the crystalline calcium hydroxyapatite with Ti. The site of the introduced Ti locally provides a chemical structure resembling that of photocatalytic titanium dioxide. The combination of the “quasi-titanium dioxide” structure and the CaHAP is advantageous in that objectionable organics can be held in appropriate contact with the photocatalytic site by the adsorptive CaHAP.
Generally, a greater amount of metal-modified apatite needs to be used for more effective antibacterial or antifouling function. However, according to the method of JP No. 2000-327315, the metal-modified apatite is obtained as white powder. Thus, when the metal-modified apatite is used in an amount sufficient enough to take effect for antibacterial purposes, the resultant appearance of the electronic device may not be attractive due to the inevitable whiteness of the metal-modified apatite. Another problem of the conventional metal-modified apatite is that the particles of the metal-modified apatite powder tend to cohere and form a number of lumps in a solvent. Application of such a lumpy antibacterial material may lead to an unacceptably bad texture of the product.